404 SIR A. GEIKIE ON THE TERTIARY [^ay 1 8 96, 



Far away from the main body of the plateau in Skye, a solitary 

 remnant, perched on the highest summit of Eaasay, bears eloquent 

 witness that the basaltic tableland once stretched far to the east of 

 its present limits. 



Some of the valleys thus excavated out of the volcanic sheets are 

 many miles long, a mile or more wide, and, from crest to bottom, 

 several thousand feet deep. The deep winding sea-lochs of Mull and 

 the west of Skye form striking monuments of this part of the waste. 



Yet, impressive as are these proofs of denudation, they are perhaps 

 inferior in this respect to the evidence furnished in the same region 

 by the great cores of gabbro and granophyre. These eruptive masses 

 must once have lain under a thick pile of basalt, for they obviously 

 belong to part of the deeper-seated mechanism of the volcanic vents. 

 Yet of this vast overlying mantle every trace has been stripped off 

 from many of these cores, while in others mere patches of it remain 

 where they were welded to the intrusive bosses by the heat of 

 eruption. 



Moreover, the cores of gabbro and granophyre have been inter- 

 sected by abundant dykes which reach the present surface of the 

 ground, even up to the crests of the mountains. It is certain that 

 the uprise of these thousands of dykes could not have taken place 

 except under cover of a great depth of rock now removed, for other- 

 wise the basalt would have rushed out from the fissures at the foot 

 of the hills and filled up the valleys, instead of rising between the 

 fissure-walls to the summits of the ridges. Not a single vestige of 

 any lava-stream younger than the gabbros and granophyres has yet 

 been discovered. It is quite possible, perhaps even likely, that the 

 post-granophyre dykes did lead to the outflow of lava here and 

 there at the surface. But any proofs of such emission have been 

 utterly destroyed in the extensive degradation which the plateaux 

 have undergone. By this process of reasoning we can demonstrate 

 that valleys in Skye and Mull 3000 feet deep have been excavated 

 out of the Tertiary volcanic series. 



Among the Faroe Islands the evidence of erosion is, in some 

 respects, even more striking. I shall never forget the first impression 

 made on my mind when the dense curtain of mist within which I 

 had approached the southern end of the archipelago rapidly cleared 

 away, and the sunlit slopes and precipices of Sudero, the two Dimons, 

 Skuo and Sando, rose out of a deep blue sea. Each island showed its 

 prolongation of the same long level lines of rock-terrace. The eye 

 at once seized k on these rock-features as the dominant element in 

 the geology and the topography, for they revealed at a glance the 

 true structure of the islands, and gave a measure of the amount 

 and irregularity of the erosion of the original basalt-plateau. And 

 this first impression of stupendous degradation only deepened as one 

 advanced farther north into the more mountainous group of islands. 

 Probably nowhere else in Europe is the potency of denudation as a 

 factor in the evolution of topographical features so marvellously 

 and instructively displayed as among the north-eastern members of 

 the Faroe group. The waste might have been as gigantic among 

 amorphous rocks, such as granites and gabbros, or even among 

 schistose masses, like the Lewisian gneiss. But in these materials 



